1. Field of the Invention
This invention relates to an apparatus for coating medical devices, such as stents, and a method of using the same.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially press against the atherosclerotic plaque of the lesion for remodeling of the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient""s vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery can develop over several months after the procedure, which can require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents can be used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents can act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty; restenosis, however, is still a significant clinical problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
Biological therapy can be achieved by medicating the stents. Medicated stents or therapeutic substance eluting stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, thrombosis remain. These events are adversely affected by the mechanical aspects of the stent such as the degree of injury and disturbance in hemodynamics caused by the stent. To the extent that the mechanical functionality of stents has been optimized, it has been postulated that continued improvements could be made by pharmacological therapies. Many systemic therapies have been tried. A challenge is maintaining the necessary concentration of a therapeutic substance at the lesion site for the necessary period of time. This can be done via brute force methods using oral or intravenous administration but the issues of systemic toxicity and side effects arise. Therefore, a preferred route can be achieved by local delivery of a therapeutic substance from the stent itself. Being made of metal, plain stents are not useful for therapeutic substance delivery. Therefore, a coating, usually made from a polymer, is applied to serve as a therapeutic substance reservoir. A solution of a polymer dissolved in a solvent and a therapeutic substance added thereto is applied to the stent and the solvent is allowed to evaporate. Accordingly, a polymeric coating impregnated with a therapeutic substance remains on the surface of the stent.
In order to be effectively applied with conventional spraying or dipping techniques, the solution needs to have a low viscosity. Low viscosities can be achieved by adding a higher fraction of solvent to the solution or by changing the composition of the solution with the addition of a wetting fluid. Compositions having a low viscosity require multiple applications of the composition and evaporation of the solvent in order to obtain a coating of suitable thickness, as compared to using compositions having greater viscosities. Accordingly, it is desired to use more viscous compositions to reduce the number of application steps and in effect reduce the processing time of forming the coating.
In accordance with one aspect of the embodiments of the invention, a method for coating stents is provided. The method can include applying a composition to a spinning disk member so the centripetal force that is applied to the composition by the disk member discharges the composition off of the disk member and onto the stent.
In one embodiment of the present invention, the method can include adjusting the temperature of the disk member to a temperature other than room temperature. The disk member can be flat, conical, or bowl-shaped. In one embodiment, the disk member can have a lip extending in an upwardly direction about the periphery thereof. The surface of the disk member can include grooves for altering the path of the composition.
In accordance with another aspect of the embodiments of the invention, an apparatus for coating a stent with a composition is provided. The apparatus includes a disk member capable of rotating about an axis of the disk member. The apparatus can also include a nozzle for applying the composition to the disk member and a motor for rotating the disk member.